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Another, better, alternative is this warning light from England. Available on E Bay for @$21.
The connection on the alternator is the "D" terminal. The downside of this installation is that it only indicates total failure of the alternator.
Problem: During flight, if the alternator failed, what would
immediately warn you of the problem? The voltmeter is by your feet, out of the normal scan area. The difference between a charging unit (@14V+-) and the battery 12V is @a needles width! Hard to notice while fully occupied piloting the helicopter.
Solution: Indicator (Idiot) Light
The red LED with leads and resistor was purchased from Auto
Zone. The mount was purchased from Radio Shack.